It is known to prepare perfluoropolyethers having perfluoroalkyl neutral end groups via photooxidation of fluoroolefins and subsequent fluorination with gaseous fluorine either in the presence or in the absence of ultraviolet radiations, thereby obtaining elimination of the peroxide groups and the conversion of the acid end groups to perfluoroalkyl groups. By this process, mixtures of products exhibiting different molecular weights and a very wide molecular weight distribution are generally obtained. The proportion of high molecular weight products is generally considerable. Processes of this type are described in U.S. Pat. Nos. 3,442,942, 3,665,041, 3,683,027, and in patent application EP 193,028.
For many applications of industrial interest it is necessary to have available products exhibiting a substantially lower molecular weight and a higher homogeneity. Thus, it was necessary to adopt methods for reducing the molecular weight, based on the thermal catalytic fractionation of the product in the presence of proper catalysts, according to patent applications EP 167,258 and EP 223,238.
This process is very important from an industrial viewpoint, because it permits one to fully utilize the photooxidation products, which generally have a high average molecular weight and may not be usefully utilized directly as such.
On the other hand, the separation of products having a relatively low average molecular weight--as is desired--from the photosynthesis rough mixture is very complicated and difficult to be carried out on an industrial scale. Furthermore, the yield of useful products exhibiting a low molecular weight is quite low (about 10%-20%).
Thus, the necessity was felt to have available an alternative industrial process capable of providing a very high yield of products, having molecular weights in the desired range, directly in the photooxidation process, without requiring any subsequent operative step for the molecular weight regulation.